The increased use of the Internet has created the demand for higher data transfer rates, and greater bandwidth. One solution to meeting this demand is the use of all optical networks having fiber optic cables carrying data in the form of light signals routed by optical switches. As used herein an optical switch operates directly on a light signal during switching rather than first converting the light signal to an electrical signal, using a conventional electronic network switch for routing the signal, and then converting the switched electrical signal back to a light signal.
In optical networks there are two types of fiber optic cables: single mode and multi-mode. A single mode fiber is a glass fiber with a diameter of 8 to 10 microns or less that has one mode of transmission—i.e., only one light signal is propagated in the fiber. In a single mode fiber, only the lowest order mode propagates at the wavelength of interest, typically 1300 to 1320 nanometers (nm) or in the 1550 nm range. Single mode fiber allows for a high data transmission rate, e.g., above 10 gigabit per second (Gb/s), longer transmission distance, and the signal distortion and attenuation is small compared to a multi-mode fiber.
While single mode fiber optic cable is readily available, all optical single mode switches are still under development. One type of all optical single mode switch is a planar electro-optical (EO) switch having a slab waveguide. Light travelling in the waveguide can be bent, when an electric field is generated by electrodes, with a voltage difference, located on opposite sides of the waveguide. One material researched for use as the slab waveguide is a transparent ferroelectric oxide, for example, lithium niobate, barium titanate, lead zircornium titanate (PZT), lead lanthanum zirconium titanate (PLZT), and strontium barium niobate (SBN). When an electric field is applied across the transparent ferroelectric oxide, the refractive index changes depending on the strength of the electric field, and hence a light signal propagating in the ferroelectric oxide material can be bent.
Although an EO switch with a thin ferroelectric oxide waveguide core with a thickness about the diameter of a single mode fiber optic cable core, e.g., 8-10 microns, should satisfy the need for an optical switch in an all optical network, there are problems in achieving this thickness. There are three processes that have been investigated, the Metal-Organic Chemical Vapor Deposition (MOCVD) process, pulsed laser deposition (PLD), and the sol-gel process. Because the sol-gel process is much cheaper, producing a thin film core by a sol-gel process is preferred. Thus there has been much research into producing a waveguide core with about 8-10 microns in thickness using the sol-gel process on ferroelectric materials. However, there have been difficulties fabricating a thin ferroelectric oxide waveguide slab over about 1 micron in thickness for large size substrates, because when the sol-gel film reaches its critical thickness, the film cracks.
Even if a thin ferroelectric oxide waveguide core with a thickness over about 1 micron could be produced, there are still other problems in using the thin core in a switch. There would be coupling problems between the fiber optic cable and the thinner waveguide. In addition, the collimating lens is typically placed external to the waveguide, and hence causes significant reflection and coupling loss of the light signal.
Therefore, as the demand for high speed optical switches grows, there is a need for an electro-optical switch having a thin film ferroelectric oxide waveguide produced from the relatively inexpensive sol-gel process, that has less of the problems associated with conventional all optical switches, and that can provide switching for high data transmission rates using a single mode light signal.